Wholly aromatic polyester resins have long been known. For instance, 4-hydroxybenzoic acid homopolymer and copolymers have been provided in the past and are commercially available. Such polymers commonly are crystalline in nature, relatively high melting or possess a decomposition temperature which is below the melting point, and when molten, frequently exhibit orientation in the melt.
The homopolymer of p-hydroxybenzoic acid is a very high melting, insoluble material and, hence, very difficult to fabricate. Melting points as high as 610.degree. C. were quoted--see W. J. Jackson, The British Polymer Journal, December 1980, p. 155. In order to depress the high melting point of the homopolymer so as to make it melt fabricable, a variety of materials incorporating different types of comonomers were prepared over the years.
One such material is, for example, the resin made from p-hydroxybenzoic acid, isophthalic and/or terephthalic acids and 4,4'-biphenol as described in, for example, Cottis et al., U.S. Pat. Nos. 3,637,595 and 3,975,487. The polymer has outstanding high temperature properties; it can be molded to give articles of high modulus and strength. It is offered commercially by Amoco Performance Products, Inc. under the trade name of Xydar..RTM. Cottis U.S. Pat. No. 4,563,508 discloses that the molding properties of a polyester can be improved by adding minor amounts of similar polyarylates in which the terephthalic acid moiety has been wholly or partially replaced by isophthalic acid. However, Cottis does not disclose or suggest the addition of a tetra polyarylate of the type disclosed herein.
Recent work at Amoco resulted in the preparation of 1-19 parts by weight of a first polyester comprising recurring moieties of dihydroxybenzene, nonvicinal benzene dicarboxylate, para-oxybenzoate to 1 part by weight of a second polyester comprising recurring moieties of diphenol, nonvicinal benzene dicarboxylate, para-oxybenzoate. The preferred mole ratio of monomers in each polyester is depected in the triangular diagrams of FIG. 1 and FIG. 2, respectively, and is described in the Definition of the Invention. The polymers melt in the range of from about 300.degree. to about 420.degree. C. Of particular interest are the first polyesters falling into the two areas A and B of FIG. 1. These latter products were found to form a stable oriented melt phase at 340.degree. to 400.degree. C.; the melt phase is tractable and can be extruded below its decomposition temperature to form high modulus and high strength fibers, films and molded articles displaying a good retention of properties at high temperatures. Materials filled with 30 weight percent of glass fibers have heat distortion temperatures of from about 240.degree. to about 280.degree. C., under a load of 264 psi. Moreover, the polymers grouped in areas A and B have crystallization temperatures in the range of from 300.degree. to 340.degree. C.; and their crystallization rates are at least 1000 counts per minute and up to 3500 counts per minute as measured by X-ray techniques. The polyesters shown in the aforementioned areas A and B are the subject of commonly assigned U.S. patent application, entitled "Hydroquinone Poly(iso-terephthalates) Containing Residues of p-Hydroxybenzoic Acid", Serial No. 225,670, filed concurrently with the present application hereby incorporated by reference.
While the overall combination of properties of the above resins is outstanding, they may form molded parts that show undesirable blistering (i.e., raised areas).
It has now been discovered that the addition of the 1-19 parts by weight of a first polyester comprising recurring moieties of dihydroxybenzene, nonvicinal benzene dicarboxylate, para-oxybenzoate to 1 part by weight of a second polyester comprising recurring moieties of diphenol, nonvicinal benzene dicarboxylate, para-oxybenzoate wherein said units are present in specified proportions yields alloys in which the tendency to blister is substantially reduced or eliminated.
With some known exceptions, mixtures of polymeric materials are generally immiscible. That is, they consist of domains of chemically distinct phases. Usually, one component forms a continuous phase, while the other component forms roughly spherical domains as inclusions. Under some circumstances, bi-continuous structures are also obtainable. Mixing two arbitrarily chosen polymers usually results in inferior materials having no utility, since in the absence of adhesion between phases, the dispersed phase merely weakens the continuous phase. Some polymeric products, such as the wholly aromatic polyesters, exhibit an ordered structure in at least some regions of the polymer. This order can exist in one, two or three dimensions. The incorporation into blends of polymers exhibiting an ordered structure leads to an increased tendency of the blends to separate into phases. This is believed to be due to the fact that the order found in certain regions of the resin causes a fairly sharp boundary between the domains of the molecules of the component polymers. Hence, blends including such polymers would be expected to exhibit a significant reduction in properties.
It should be noted, however, that many useful blends whose morphology and phase interaction are favorable, are known.
Takayanagi et al., U.S. Pat. No. 4,228,218, disclose a polymer composition comprising 20 percent or less, based upon the total weight of polymeric material, of a first rigid polymer with the balance being a second polymer composed substantially of flexible molecular chains. An approximate borderline between rigid and flexible molecular chains is a Mark-Houwink Index of 1, those above being rigid and those below being flexible. The first polymeric material is dispersed in the second polymeric material in a microscopic region of 1 .mu.m or less. It is believed that wholly aromatic polyesters would be characterized by those skilled in the art as rigid molecules within the context of the above cited patent. The patent does not disclose blends of two or more polymers having rigid chains as does the present invention with improved blister resistance.
Blends of polymers exhibiting orientation in the melt with other polymers were investigated. Mixtures of liquid crystalline polyesters with poly(alkylene terephthalates), polycarbonates and polyarylates were described in Cincotta et al., U.S. Pat. Nos. 4,408,022 and 4,451,611; Froix, U.S. Pat. Nos. 4,489,190 and 4,460,735; and in Kiss, European Patent Application No. 169,947. Improved mechanical properties were found with these materials. The addition of a particulate liquid crystal polymer to poly(butylene terephthalate) or other thermoplastic polymers was described as a method to obtain compositions with enhanced resistance to melt dripping during burning--see Kim et al., U.S. Pat. No. 4,439,578. In several instances, e.g. in alloys of liquid crystalline polyesters with an aromatic sulfone polymer (Froix et al., U.S. Pat. No. 4,460,736) with an aromatic poly(ester amide) (Kiss, U.S. Pat. No. 4,567,227), and with poly(arylene sulfides) (Froix, U.S. Pat. No. 4,276,397) improved mechanical characteristics and improved processibility (lower viscosity) of the non-anisotropic resin were noted. Better properties were also obtained by blending two particular liquid crystalline polyesters--see, for example, Froix, U.S. Pat. No. 4,267,289.
Liquid crystalline materials, including polyesters, were used to decrease the viscosity and improve the processibility of a number of other resins, including fluorinated polyolefins--see Bailey et al., U.S. Pat. No. 4,417,020; Cogswell et al., U.S. Pat. Nos. 4,429,078 and 4,438,236; and George et al., U.S. Pat. No. 4,650,836.
In one instance (Baily et al., U.S. Pat. No. 4,508,891) it was claimed that the addition of an isotropic resin to an anisotropic resin leads to a decrease of anisotropy in the corresponding molded articles.
No reference is known which is directed to the improvement of surface properties (i.e. blistering) by blending two polymers having orientation in the melt. As indicated earlier, alloys of materials having ordered structures would be expected to have reduced properties. Hence, the instant discovery was highly surprising and totally unexpected.